@article{sun2018u,
  title={U-net: Machine reading comprehension with unanswerable questions},
  author={Sun, Fu and Li, Linyang and Qiu, Xipeng and Liu, Yang},
  journal={arXiv preprint arXiv:1810.06638},
  year={2018},
  url={https://arxiv.org/pdf/1810.06638}
}